Add ring and promote methods for Constant objects

Add support for creating matrices with Constant entries.  Also add a
unit test.

M2/Macaulay2/m2/reals.m2

@@ -270,6 +270,10 @@ EulerConstant = new Constant from { symbol EulerConstant, mpfrConstantEuler, eRR
 CatalanConstant = new Constant from { symbol CatalanConstant, mpfrConstantCatalan, cRRi0}
 ii = new Constant from { symbol ii, ConstantII}
 
+ring Constant := ring @@ numeric
+promote(Constant, InexactNumber') := (x, kk) -> (
+    promote(numeric(precision kk, x), kk))
+
 isFinite Constant := x -> true
 
 lngamma = method()

M2/Macaulay2/tests/normal/matrix.m2

@@ -371,6 +371,9 @@ scan({matrix {}, matrix(ZZ, {}), map(ZZ^0, ZZ^0, {}), map(ZZ^0,, {})}, A -> (
 	    assert Equation(source A, ZZ^0);
 	    assert Equation(target A, ZZ^0)))
 
+-- issue #3456
+assert Equation(matrix {{ii}}, matrix {{numeric ii}})
+
 -- Local Variables:
 -- compile-command: "make -C $M2BUILDDIR/Macaulay2/packages/Macaulay2Doc/test matrix.out"
 -- End: